Groupe de travail NAGANA

Préséntation

 Intitulé Numerical Algebraic Geometry and Algebraic Numerical Analysis Organisateurs Michele Bolognesi et Daniele A. Di Pietro Objectifs Ce groupe de travail a pour but d'explorer des développements récents au carrefour entre la géométrie algébrique et l'analyse numérique. Au passage, il permettra de renforcer les liens entre les équipes de mathématiques fondamentales et appliquées de l'IMAG. Structure Deux-trois séances par an animées par des orateurs internationaux de très haut niveau Acknowledgements ANR projects HHOMM and fast4hho EDF

Historique des exposés

• 9–11 March 2020, Jonathan Hauenstein, University of Notre Dame

To be announced

• 24 January 2020, Salle 330 9h-11h Francesca Rapetti, Université Côte d'Azur

High order Whitney forms on simplices
Whitney elements on simplices are perhaps the most widely used finite elements in computational electromagnetics. They offer the simplest construction of polynomial iscrete differential forms on simplicial complexes. Their associated degrees of freedom (dofs) have a very clear physical meaning and give a recipe for discretizing physical balance laws, e.g., Maxwell’s equations. As interest grew for the use of high order schemes, such as hp-finite element or spectral element methods, higher-order extensions of Whitney forms have become an important computational tool, appreciated for their better convergence and accuracy properties. However, it has remained unclear what kind of cochains such elements should be associated with: Can the corresponding dofs be assigned to precise geometrical elements of the mesh, just as, for instance, a degree of freedom for the space of Whitney 1-forms belongs to a specific edge? We address this localization issue. Why is this an issue? The existing constructions of high order extensions of Whitney elements follow the traditional FEM path of using higher and higher “moments” to define the needed dofs. As a result, such high order finite $k$-elements in $n$ dimensions include dofs associated to $q$-simplices, with $k < q \le n$ , whose physical interpretation is obscure. The present presentation offers an approach based on the so-called “small simplices”, a set of subsimplices obtained by homothetic contractions of the original mesh simplices, centered at mesh nodes (or more generally, when going up in degree, at points of the principal lattice of each original simplex). Degrees of freedom of the high-order Whitney k-forms are then associated with small simplices of dimension k only. We provide an explicit basis for these elements on simplices and we justify this approachfrom a geometric point of view (in the spirit of Hassler Whitney's approach, still successful 30 years after his death).

• 23–24 October 2019, Salle 109 Bernard Mourrain, INRIA (AROMATH project) [slides]

Spline functions for geometric modeling and numerical simulation
In geometric modeling, a standard representation of shapes is based on parameterized surfaces or volumes, which involve piecewise polynomial functions also known as splines. We will first describe these splines, in the univariate case, how they are characterized and what are their main properties. We will present some extensions to higher dimensions and how they are used for modeling shapes, in several domains of applications. We will also illustrate several exemples of spline constructions, that have been investigated. In the last decades, a new paradigm called isogeometric analysis has emerged, which uses splines functions not only to describe the geometry but also to approximate functions on this geometry. We will briefly describe how this approach is working in numerical simulation, what are its main characteristics and we will provide some illustrations. Analyzing the spaces of spline functions associated to a given domain partition or a given abstract topology is an important problem for geometric modeling and numerical simulations. We will present some of the know results and some problems still open. We will describe algebraic-geometric techniques, which provide a better insight on these spline spaces, in particular for the analysis of their dimension and for the construction of basis. This analysis involves topological complexes and homological tools, which will be explained by examples. Spline spaces over triangular meshes or T-meshes, in dimension 2 and 3, as well as extensions to geometrically smooth spline spaces (if time permits) will be considered in details.

• 2 April 2019, Salle 440 Daniele Boffi, Università di Pavia [slides]

From the finite element method to the finite element exterior calculus
The finite element exterior calculus (FEEC) has been developed in the early 2000s as a mathematical framework that uses the calculus of differential forms for the formulation of the finite element method. The aim of this lecture is to revisit some of the main aspects of FEEC. We shall use as a main motivation the approximation of Laplace equation in mixed form and of the time harmonic formulation of Maxwell's equations. It is remarkable that, besides providing a more elegant and comprehensive understanding of the existing theory, the FEEC allows for the development of new finite elements and for the solution of problems that would be difficult, if not impossible, to address with more traditional tools

• Updated 21/1/2020